1. Field of the Invention
The present invention relates to a soldering method for mounting electronic components on a printed circuit board using a solder paste patterned on the circuit board. Particularly, the present invention relates to improvement of soldering flux which is applicable in wide range of soldering temperature, and enables to provide a low temperature solder paste for mounting semiconductor chips.
2. Description of the Prior Art
Recently, electronic components including semiconductor chips are mounted on a printed circuit board by soldering using a solder paste which is patterned on the printed circuit board applying various technology of thick film circuit such as silk screen printing, for example. Then, circuit elements are positioned on respective positions to be soldered, and the printed circuit board is heated in an oven to solder the circuit elements.
In such case, it is desirable to prepare several kinds of solder paste each including solder alloy powder having different melting points. They are properly used to solder various circuit elements on the circuit board. Especially, it is necessary to use a low temperature solder paste for soldering semiconductor chips in order to prevent deterioration of the chips. Usually, semiconductor chips are soldered using a low temperature solder paste after all other circuit elements such as capacitors or resistors have been mounted. Such soldering method utilizing the difference of melting point of solder is called in the art as step soldering.
The solder paste is composed of powder of solder metal kneaded with flux. The melting point of the solder paste is determined by the melting point of solder powder, which is determined by mixing rate of content materials such as lead, tin, gallium, indium etc. The flux is used for removing oxide films on the surface of the metal to be soldered and for preventing oxidation of the solder during soldering operation.
It is necessary to prepare various fluxes suitable for respective solder powders. Flux includes an activator which has a function to reduce and eliminate the oxide film covering the surface of soldering region. Usually, amine hydrogen halide is used, as an activator for liberating hydrogen halide by heat processing for soldering. Ordinary activator cannot operate sufficiently if it is applied in small quantity especially when the activator is used for low temperature soldering. Accordingly, diethylen amine hydrogen chloride of 0.3 weight percent of chlorine is usually added to the flux, but this amount often causes a deterioration of the soldered portion.
If the solder paste is stored for a long time, it increases its viscosity and becomes unsuitable for screen printing. Therefore, the present state-of-art solder paste has a shelf life, that means a period of time during which it should be used. The shelf life for ordinary high melting point solder paste is less than a week, however, the shelf life for low melting point solder paste becomes shorter as the melting point becomes low. And a solder paste having a melting point of 117.degree. C., for example, which is suitable for soldering semiconductor chips has a very short shelf life of only on day.
The short shelf life is very inconvenient for practical use, and it is a defect for soldering works of printed circuit board manufacturing. Even within a period of the shelf life, the solder paste having short shelf life varies its characteristics, such as hardness or viscosity for example, from hour to hour. Accordingly, it must be done very carefully to handle and preserve the solder paste. In many work shops of the printed circuit board manufacturing, the solder paste has to be prepared daily, and it is kept in refrigerator to maintain constant characteristics of the solder paste. Moreover, it is necessary to prepare various kinds of flux suitable for each of different melting point solder. It has been a very troublesome work for the workshops.
Even though the solder paste is preserved carefully, deviation of thickness of the silk screened pattern of solder paste often occurs, it is attributed to the variation of characteristics of the solder paste, and reliability of soldering is decreased. These are ascribed to the short shelf life.